mmx register moves through memory

[Vahur Sinijärv]

Richard Henderson wrote:

>>I do not see the point why you should discourage the register allocator 
>>from using mmx registers, move through memory is clearly inefficent and 
>>enlarges resulting code (if the function containing moves is inlined in 
>>several places, even more so).
>>    
>>
>
>First, what you think is "clearly inefficient" is at least two cycles
>faster, at least for AMD (Intel hasn't published anything as useful as
>instruction latencies since early PentiumPro).  I'm not sure what sort
>of pipeline bypasses are or are not responsible, but *all* cross function
>unit moves are discouraged.
>
>  
>
I see, i did a speed test with GCC 4.1.0 and 3.4.4 on my athlon-xp and 
you are right. Direct moves between genregs and MMX are still useful 
when optimizing for size. GCC could do a bit more sophisticated guesses 
whether to use secondary memory for such moves or not, right now it just 
disables them all.

>Second, proper use of MMX requires proper placement of emms instructions.
>Allowing the register allocator to use MMX registers at will breaks that.
>
>
>r~
>  
>
That is true, but would register allocator choose MMX regs for anything 
else than MMX ops ?

Regards,
Vahur